Revenues Deferred Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Deferred Revenue Rollforward [Line Items]
Document Period End Date	Mar. 31, 2018
Deferred Revenue	$ 250	$ 194
Deferred Revenue, Additions	419
Deferred Revenue, Revenue Recognized	363
Prepaid Rentals [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	155	101
Deferred Revenue, Additions	367
Deferred Revenue, Revenue Recognized	313
Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	95	$ 93
Deferred Revenue, Additions	52
Deferred Revenue, Revenue Recognized	50
Accounts Payable and Accrued Liabilities [Member] | Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	39
Other Noncurrent Liabilities [Member] | Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 56